This invention relates generally to improved aqueous emulsions of phenolic resins for use as color developing agents for pressure-sensitive and heat-sensitive copying papers and improved methods for their manufacture.
In the process of preparing phenolic resins for color developing agents, the usual procedure, heretofore, comprised the steps of reacting a phenol with an aldehyde in the presence of an acidic catalyst to produce a precondensate, subsequently heating to concentrate the precondensate at elevated temperatures of from 130.degree. C. to 200.degree. C., thereafter cooling the concentrated precondensate to obtain solid lumps of the phenolic resin. The solidified resin was then mechanically pulverized, first by means of a dry pulverizer into finely divided particles, mixing the resultant fine particles with water, a dispersing agent, surfactant and other additives, followed with treating the mixture to wet pulverization, such as in a colloid mill, sand grinding mill or attriter to obtain an emulsion.
As a modification of this earlier procedure, the resin would be dissolved in an organic solvent wherein the solution was then converted into an oil-in-water emulsion.
However, various problems arose when the precondensate was heated, namely increased molecular weight, lowered solubility in dye solutions, impaired color-developing properties (speed in color development and density of color) and degraded fastness to light, which rendered such earlier methods less than satisfactory.
In addition to the above shortcomings, the step of subjecting the phenolic resin to dry pulverization was time consuming and polluted the working environment raising the possibility of dust explosion.
It has now been discovered that superior emulsions with excellent color developing properties and fastness to light may be prepared with phenolic resin particles having a fine, uniform diameter and relatively low molecular weight distributed within a narrow range. In addition, an improved method for preparing such phenolic resins for color developing emulsions has been discovered which eliminates the need for the steps of high temperature concentration and dry pulverization.
Accordingly, it is the principal object of the present invention to provide improved color developing emulsions for copying papers that have superior color developing properties, fastness to light and extended shelf life.
It is a further object of the present invention to provide improved processes for preparing phenolic resins for color developing emulsions by an in-situ type method.
A still further object of the instant invention is to provide improved methods for preparing non-toxic, non-flammable aqueous phenolic resin-containing color developing emulsions through the elimination of elevated temperature concentrating and dry pulverization.
These and other objects, features and advantages will become apparent to those skilled in the art after a reading of the following more detailed description.